DE2554458A1 - METHOD AND DEVICE FOR DETERMINING THE PROPERTIES OF THE EARTH MATERIALS SURROUNDING A DRILL HOLE - Google Patents

Description

Patent assessor Hamburg, Nov. 24, 1975

Dr. Gerhard Schupfner 770 / kr

Deutsche Texaco AG _φ ", _Λ , *

Patent department ^{Τ 75 041 D} < ^D 73,798-F)

2000 Hamburg 13

Middle way 180

TEXACO DEVELOPMENT CORPORATION 135 East 42nd Street New York, N.Y. 10017 UNITED STATES.

Method and apparatus for determining the properties of the earth materials surrounding a borehole

The invention relates to a method and a device for determining the properties of those surrounding a borehole Earth materials; In particular, the invention relates to a method and an apparatus for combining a High frequency dielectric induction logging using a conventional low frequency induction logging to based on the measurements hydrocarbon-bearing formations of fresh water-bearing formations to be able to distinguish in porous earth formations.

For many years it has been common practice to measure the electrical properties of the rock formations in the area of a borehole for the purpose of determining the location of an oil-bearing formation

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to eat. This measurement was made possible by using an electrical resistance measuring device in bores with a highly conductive borehole fluid and through the use of an induction measuring device in bores that go through Use of an oil-based drilling mud or a drilling fluid that has a higher specific resistance, were created. In conventional resistance measuring probes, a current-emitting electrode (or an electrode arrangement) arranged to focus the emitting current, the emitting either a direct current or a low-frequency (e.g. 60 Hertz) alternating current into the earth formations surrounding the borehole via contact electrodes to be used. These currents penetrate part of the earth's formations and are generated by a current electrode, which is arranged at a certain distance from the emitting electrode is determined. The size of the determined currents can then be used to determine the resistivity of the earth formation surrounding the borehole. In some Cases were current electrodes together with potential measuring electrodes used to determine the resistivity in the formation.

An electrical induction borehole survey was carried out in the past by means of a probe, which has a transmitter coil (or an arrangement of such coils) and at some distance from it a receiver coil (or an arrangement of receiver coils). Usually it was a

high-frequency alternating current passed through the transmitter coil (about 20 kilohertz). The resulting electric field within the earth formations was removed from the something arranged receiver coil determined by the induced current or the voltage in the receiver coil was measured. The usability of the two above-described borehole measuring systems results from the fact that earth formations with a pore space which is filled with a hydrocarbon have a higher specific resistance as earth formations, the pore space of which is filled with either salt water or some other conductive fluids.

Difficulties arose in the interpretation of the measurement results when the resistance measurements in freshwater formations (relatively non-conductive when less than 10,000 ppm sodium chloride is present). Such fresh water Sands or earth formations exhibiting a high specific resistance, which is similarly high as in hydrocarbon-bearing formations. In these cases it is difficult, if not impossible, to base on the electrical or induction readings alone determine whether the formation is a freshwater formation or a hydrocarbon-bearing formation. Because of this, there is a great need for a measuring system in which, on the basis of some physical properties, the earth formation surrounding the borehole is distinguished whether the formation is a

609825/0693

Freshwater or hydrocarbon-filled earth formation acts.

The object of the invention "is to provide a method and to create a device with the fresh water leading formations of hydrocarbon leading formations in the Area of a borehole by combining a conventional induction measuring technique with an inductive dielectric measuring technique can be distinguished.

The object of the invention is achieved by providing a borehole measuring system which is a combination of a high frequency dielectric induction measuring system with a

represents

conventional depth induction logging system / i In the Measuring probe of the present invention, two different measuring apparatuses are provided. One measuring apparatus is a High frequency dielectric induction measuring apparatus, the one

singles
16 MHz "" v-transmitter coil together with a 16 MHz receiver coil, which is arranged remotely from the transmitter coil. The second induction measuring apparatus comprises a conventional 20 kilohertz depth induction measuring apparatus, as is known from the art. With the high frequencies used in the dielectric induction measurement method, the. high-frequency alternating currents induced in the formation influenced by the dielectric constant and the conductivity or the specific resistance of the formation in the area of the borehole.

608828/0698

. By measuring the received

Signal at each of these two widely spaced frequencies and, through the composition, they sang accordingly According to the invention, oil-bearing zones can be distinguished from fresh water-bearing zones, which by interpretation the measurement results of conventional techniques have been difficult.

An exemplary embodiment of the invention, from which further inventive features emerge, is shown in the drawings. Show it:

Fig. 1 is a block diagram of the device according to the invention

Fig. 2 is a schematic block diagram of the underground guided electronics of the measuring probe

Fig. 3 is a graph showing the relationship between the measurement result of the conventional depth induction measurement and high frequency dielectric measurements according to the system of the invention.

4 shows a theoretically derived graphical representation the change in amplitude of a 64 MHz high frequency induction dielectric single probe for drill holes with a diameter of 0 - 0.35 m.

5 shows a theoretically derived graphical representation of the change in amplitude of a 130 MHz high-frequency induction dielectric single probe for drill holes

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with a diameter of 0.0 - 0.4 m.

Hydrocarbons have a characteristic low dielectric constant ί _r , which is less than 5. On the other hand, fresh water has a relatively high dielectric constant _r of around 50%. The absolute dielectric constant of a material £ is defined as the natural electrical polarization of this material. In the description below, the expressions

Konsiiän1j6 "» relative dielectric constant and "dielectric constant" C _r used synonymously. These quantities are related to the absolute dielectric constant of free space ζ _Q according to the following equation

In this equation, £ _{Q is given} as 8.854 pF per meter.

Based on the electromagnetic field theory and especially the theory of a point-like oscillating magnetic dipole, the behavior of the high-frequency field in the area of a cylindrical borehole can be represented according to the following equation 2 (Helmholiz equation with the cylindrical coordinates η , ψ and Z ).

^(m)

ωμ "Ρ

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In this equation ff ^ ^m 'means the Hertzian magnetic vector? I ^ ^m 'the magnitude of the current; j = V "-? · The

complex chung 2 represented quantity K is the / circular wave number that

by equation 3 explicitly reads as follows:

2 2

(3)

In equation 2, the expressions dtp \ and <^ ( ₂ ) are individual momentum functions of the Dirac type. O is equal to 2 iTf, where f is the oscillation frequency of the point magnetic dipole and I is the dielectric constant of the medium. JtA- denotes the magnetic dielectric constant of the material surrounding the magnetic dipole and G ^ the electrical conductivity of the medium.

From equations 2 and 3 it can be seen that three physical constants are to be considered with the material surrounding the point magnetic dipole. These three constants are the relative magnetic permeability Lt,

di- _r ^/

the electrical material constant L and the electrical conductivity Qf. For most earth formations, the relative magnetic permeability JU at the given frequencies (10 - 60 MHz) can be taken as a constant. Variations in this value fall in the range of 0.001-0.1% for earth materials. The only values of interest that remain are £ and (T · These two physical properties have a direct influence on any high-frequency alternating current within the media.

609826 / neef »

The properties of the medium have an effect on the size and phase with respect to those induced by the transmitter coil or the stray currents within the formation in the area of the borehole.

Assuming that it is a point-shaped magnetic source, which is arranged in a cylindrical borehole is and with reference to the Helmholtz equation, For example, the total field can be defined as the field of the source detected by the receiver coil in any medium will. The total field can be divided into a primary field and a secondary field; the primary field is defined as the field of a source that is received by a receiving coil in a Reference medium (e.g. vacuum or air) has been determined. The secondary field is defined as the field that is used in vectorial Adding to the primary field results in the entire field. The primary field has an amplitude and a phase equal to the amplitude and phase of the total field in a comparison medium. If the source is placed in a medium that differs from the reference medium differs, the secondary field is added to the primary field and thus creates the entire field within of the new medium. The primary field serves as an amplitude and phase comparison for determining the secondary field. the

the

in the / oscillating punctiform magnetic dipole surrounding Medium flowing currents are called stray currents. The stray currents generate secondary fields, which in the case of a highly conductive medium counteract the primary or comparison field. However, if the value of ο ("(the

Angular frequency multiplied by the dielectric material constant) the size of the value Qf (the electrical conductivity), the stray currents are phase-shifted and can actually occur in the secondary field, which increases the overall field. This is usually the case when working at frequencies that e.g. B. have frequencies in the range of 10-60 MHz.

Since changes in the values [ and G "cause changes in the stray currents at any given frequency, the separation of the two effects cannot be done by measuring a single voltage amplitude of the field The above-mentioned high frequency together with the value of the specific resistance of the formation determined using a conventional deep induction measuring device, if these values are appropriately combined, a simultaneous derivation of the two values (and (Verölgen.

The Helmholtz equation (equation 2) applies in every cylindrical layer of a layered medium, which is the point-shaped surrounding magnetic dipole in the borehole. When using a computer program to carry out numerical integration of the solutions of equation 2 in different cylindrical positions around the dipole and when boundary conditions are applied to the At the interface of these regions and at the source, this can be a distance ζ along the borehole axis from the magnetic

Dipole-shifted total field derived from the receiver coil will.

By examining the numerical solution of the Helmholtz equation (Equation 2) in boreholes of different diameters, graphical representations of the field amplitude can be displayed the receiver coil as a function of the borehole diameter for different sizes can be made. Such a graphic representation is in FIG. 4 for a 64 MHz single transmitter coil and receiver system and shown for a 32 MHz dual transmitter coil and single receiver coil system. From Fig. 4 it can be seen that at a 64 MHz frequency an abnormal resonance effect occurs at a borehole radius of about 254 mm. It can also be seen that the 32 MHz dual encoder coil amplitude frequency does not show this resonance effect with adequately large boreholes.

Fig. 5 shows a graph of the normalized Total field amplitude at the receiver coil on the z-axis of the borehole as a function of the borehole radius for an operating frequency of 130 MHz. In this case, the reso-

approximately

nanze effect with a borehole radius of / 100 mm and again with a borehole radius of about 250 mm. So should attempts are made to measure the dielectric and conductivity properties of the materials surrounding the borehole at frequencies to measure at a height of 64 MHz, it can be seen from the graph in FIGS. 4 and 5 that some correction

ren for the resonance effects are necessary. On the other hand, it is supposed to be remembered that for the purpose of determining the electrical conductivity and the material constants (or the dielectric Constants) of the materials surrounding the borehole at least two independent measurements for the purpose of interpretation of the resulting measurements in values of the dielectric constant / and the conductivity G ^ of those surrounding the borehole Materials need to be carried out.

To the resonance effects shown with reference to FIGS to avoid, in the present invention the amplitude measurements of the total field signal at a receiver coil, which is guided inside the borehole, carried out at frequencies of about 10 - 60 MHz. It was found,

that measurements at the aforementioned frequency are more accurate than those available at higher frequencies due to the borehole resonance effects. In a special embodiment of the invention, the measurements are made at one frequency of 16 MHz and with a conventional induction measuring apparatus at a frequency of approximately 20 KHz.

In Fig. 1, the combined measuring device is shown schematically. A measuring probe 11, whose probe body is preferably made of

non-fiberglass material or some other / conductive material with sufficient Strength properties is made, depends on a measuring cable 12 in an uncased borehole 13. The drilling

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Hole 13 is filled with a borehole fluid 14 and surrounded by earth formations 15, the properties of which are measured should.

The upper part of the measuring probe 11 has a conventional depth induction measuring device 18, which operates at a frequency of approximately 20 KHz. The technical basics and functionality of the aforementioned induction measuring apparatus for measurement the resistivity of the formation (or the conductivity) is known per se to the person skilled in the art and needs within the framework not to be discussed in further detail in this application. As a source of literature, the "Schlumberger Log Interpre-

Principles
tation ", Volume I, / Edition 1972, published by Schlumberger Limited, 277 Park Avenue, New York, USA. For the purposes of this application, it is sufficient that the depth induction resistance measuring device 18 generates an output signal , which is representative of the specific resistance of the earth formations 15 in the area of the borehole 13, which is appropriately multiplexed with the high-frequency dielectric induction measurement to be described below, for the purpose of transmission via a conductor of the measuring cable 12

transfer .
to the surface of the earth / can. (In this context, reference should be made to a parallel application by the applicant P 25 4-7 834.6 - T 75 040.)

The lower part of the borehole probe 11 has an electronic high-frequency transmitter part 16, which will be described in more detail below.

608826/0618

is explained in more detail and an associated transmitter coil 17. The transmitter coil 17 is wound into a central winding mandrel 20, which is preferably made of a non-conductive material such as fiberglass. The encoder coil 17 operates at a frequency of 16 MHz. A receiver coil 21 is at an axial distance along the axis of the downhole probe 11 arranged away from the transmitter coil 17. The distance from the center of the 16 MHz transmitter coil 17 is approximately 1 m. For the person skilled in the art it is evident that other coil distances between transmitter and receiver coils are possible and are within the scope of the invention. Other high frequencies in the frequency range of 10 - 60 MHz are also within the scope of protection the invention.

The radial depth of investigation of the logging system is from The distance between the transmitter and receiver coils is influenced. Generally can be said, the greater the distance between the encoder and the receiver coil, the greater the radial depth of investigation in the earth formation. It should be noted, however that it is necessary to arrange the transmitter and receiver coils close enough together to receive useful signal levels to ensure earth formations of different dielectric and conductive properties. As a highly conductive Material in the borehole area that attenuates high-frequency signals, it is necessary to add more energy to the encoder part use when the distance between the transmitter and receiver coils is increased.

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A = conventional winch arrangement (not shown) is on the earth's surface provided for the movement of the borehole probe 11 through the borehole during the measurement work. A roll of 22, over which the heating cable 12 is passed, can be electrically or mechanically connected to a data recording device 24- be. The recording device 24 is used to record the Signals from the underground measuring probe 11 are used and recorded as a function of the depth of the borehole 13. The for the measuring work of the receiver electronics 36 arranged in the borehole probe 11 is required via lines in the Measuring cable 12 supplied from an energy source 28 arranged above ground.

It can be seen from FIG. 1 that the signals from the measuring probe are fed to a signal isolating circuit 29 via the measuring cable 12. These input data can be viewed as two separate signals at two different intermediate frequencies f * and fp of the measuring cable. The signal separating circuit 29 separates these two signals and feeds the high-frequency part to an HF dielectric induction detector 31. The conventional depth induction signal is fed to a depth induction detector 30. The detectors 30 and 31 determine the amplitude of these two signals and transmit representative output signals therefrom to the recording device 24, which records these signals as a function of the depth of the borehole. These signals can be interpreted to provide an indication of an oil-bearing zone, which is described in more detail below

is going to attain.

In Figure 2 the high frequency dielectric part is the system shown in more detail in block diagram form. The transmitter device includes a crystal-controlled 16 MHz Oscillator 4-1. The output signal of the 16 MHz oscillator 41 is fed to a gain controlled amplifier 42, which amplifies the signal to a value and transmits it as an input signal to an output amplifier stage 43. the

Signal output amplifier stage 43 transmits a constant 16 MEz / to the transmitter coil 17 · A part of the output signal of the amplifier stage 43 is from a toroidal pickup coil 47, which is located in the Output stage of the amplifier 43 is arranged, checked. This signal is compared with a reference signal that from a voltage source 46 in a reference working amplifier stage 44 is fed. The output signal of the reference amplifier stage 44 is fed to a control amplifier stage 45, in which it is amplified linearly to a level sufficient to control the gain of amplifier stage 42. Thus, by monitoring the output of the amplifier stage 43, the high-frequency electromagnetic flux in the transmitter coil 17 by comparing with the reference signal of the reference voltage source 46 held at a constant level.

The 16 MHz receiver coil 21 is connected to a mixer 48, which is also supplied with a constant frequency input signal from an oscillator 49. The mixed

effect of the mixer 4-8 generates an output signal at a the intermediate frequencies f ,. or f ^ »those used to transmit the Data on the earth's surface via the measuring cable 12 can be used. This intermediate frequency signal becomes linear through an audio frequency amplifier 50 amplified and ultimately amplified by an amplifier 51 to a level which is sufficient for the Transmission to the surface processing system of the signals. The receiver circuit is of a conventional one in the measuring probe guided current source 52 applied to the by the

supplied above-ground power source 28 / on a corresponding Converts voltage value. The transmitter circuit is powered by a battery (not shown) in the lower part of the probe 11 is arranged, acted upon.

The armored measuring cable which is used for the present invention is designed as a triaxial measuring cable, from from the outside to the inside, an outer reinforcement, an insulation layer, comprises a coaxial cable layer, a second insulation layer and a central conductor. For the professional it can be seen that other multi-conductor measuring cables can also be used.

3 shows a graphic representation of a measurement result, which was obtained by means of the measuring probe in a homogeneous medium. The conventional induction resistance value is on the The ordinate is plotted, while the normalized one is plotted on the abscissa

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Field amplitude of the 16 MHz receiver signal is plotted. In this case, normalized is understood to mean that the receiver signal is plotted in relation to the corresponding air value will. In other words, the received 16 MHz signal is divided into the formation at the receiver coil is determined by the amplitude of the 16 MHz receiver signal at the receiver coil suspended in air.

The curves in Figure 3 are a function of both the conductivity and the electrical excitation properties of the homogeneous Medium in which the probe was embedded for the purpose of graphic representation. It can be seen from Fig. 3 that the amplitude of the 16 MHz signal has a greater dependence on the dielectric constant of the Erdformatxonsstoffe than a Has conventional induction measurement signal.

From the study of Fig. 3, an estimate can be made, that oil zones can be distinguished from fresh water zones with similarly high resistance values in the representation, if they are in the area to the left of the curves that one "anticipates-

be

visibly hydrocarbon "/ can fall. This effect occurs due to the dependence of the 16 MHz high frequency signal amplitude to the dxelectrxcitation properties of the earth formations. Curves as shown in Fig. 3 can for precise calibration for the system in formations with known resistance values and known Dxelectxcitation properties can be used, and thus to derive quantitative

604826/066 *

ver values for the oil content in unknown formations of the zu exploring oil field are used.

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Claims (8)

Claims 1.) Method for determining the properties of the earth materials surrounding a borehole, in particular for localization of hydrocarbon-bearing zones, characterized in that a high-frequency, electromagnetic Field is generated in a frequency range of 10 to 60 MHz, that at the same time a low-frequency, time-varying electromagnetic within the borehole Field is generated in a frequency range below 50 KHz, that in one of the field generator location of the high frequency field remote location, the total field amplitude of the high-frequency field is determined and a this is a function of the conductivity and the dielectric constant of the earth formation in the area of the borehole a first signal representing the amplitude value is generated, that in one of the field generator location of the low frequency field remote location determines the amplitude of the low-frequency electromagnetic field and a second signal corresponding to this conventional induction amplitude value known per se, one measurement of the conductivity of the earth formations ORIGINAL represents in the area of the borehole, is generated
and that composing the first and second signals according to a predetermined relationship to derive indications of an oil bearing zone being penetrated by the wellbore
will. 2.) The method according to claim 1, characterized in that the method steps in one
A plurality of depths within the borehole are repeated and that the measurements are recorded as a function of the borehole depth. 3.) The method according to claim Λ or 2, characterized in that the high-frequency electromagnetic field in a frequency range of 15 to
35 MHz is generated. 4.) Method according to one of the preceding claims, characterized in that the high-frequency electromagnetic field with a frequency of
16 MHz is generated. 5) Method according to one of the preceding claims, d a - characterized in that the low frequency electromagnetic field has a frequency of approximately 20 KHz is generated. 6.) Method according to one of the preceding claims, characterized in that the total field amplitude of the high-frequency field is normalized in relation to the air value of the total field. 7.) Method according to one of the preceding claims, characterized in that for simultaneous Transmission of the first and second signal to a signal system arranged above ground, this with two different intermediate frequency signals are connected. 8.) Device for determining the properties of the earth materials surrounding a borehole, in particular for localization of hydrocarbon-bearing zones, characterized by a device (17) within of the borehole to generate a high-frequency electromagnetic field in a frequency range from 10 to 60 MHz, by a known second device (18) within the borehole for the simultaneous generation of a low-frequency electromagnetic field in a frequency range below 50 KHz, by one of the generator • Location of the high-frequency field in the longitudinal direction of the borehole remote location arranged device (21) for determining the total field amplitude of the high-frequency 609825/0693 r Il t th electromagnetic field and generation of a signal corresponding to this amplitude, by a detection device for the amplitude of the low-frequency electromagnetic field and generation of one of these amplitudes corresponding second signal, by means for the simultaneous transmission of the first and second signals downhole to the surface and through a recording device (24) for recording the signals as a function the depth of the borehole. 9 ·) Device according to claim 8, characterized in that the device for simultaneous transmission of the first and second signals, a device (36) for converting the detected signals into two different ones Intermediate frequency signals for transmission showing after days. 60982S / 06SS